Police said the cyclist was travelling north on Cumberland St when the accident happened about 10am.
Police have located the truck and trailer at Evansdale, 25km north of Dunedin, and spoken to the driver.
It is unknown whether the driver was aware there had been an accident.
Serious crash investigators and other police staff were at the scene and police were working to identify the cyclist.
Police are seeking information from anybody who may have seen the incident. They could call Dunedin Police on 471-4800 or provide information anonymously via Crimestoppers on 0800 555 111.
Detours were in place around Cumberland St, where the crash occurred.